Solar cells convert photons into electrical energy using semiconductors. In semiconductors, a valence band and a conduction band of electrons are separated by an energy gap that depends on composition. When a photon is absorbed, an electron is increased in energy and moves from the valence band into the conduction band. The hole that is created in the valence band then acts as a positive charge carrier. A p-n junction in the material permits current flow in one direction but restrains it in the opposite direction, making possible conversion of photon absorption into electrical current.
The market for solar cells for use in space, particularly for satellites and vehicles operating in low-or medium-earth orbit, has increased greatly in recent years. Just the global telecommunication satellite niche represents a market size of $600M-$1 Billion over the next 5 years (Meyer, M. and Metzger, R. E.,"Compound Semiconductors," 1997) (300 satellites with a power average requirement of 1 KW /satellite with the price of solar panels approaching $2000-3000 per watt). Currently, tandem cells have 30% of that market. A tandem solar cell includes at least two cells--a cell that receives light first and absorbs higher energy photons and another cell that receives light transmitted through the first cell and absorbs lower energy or longer wavelength radiation. The cells may be connected by a tunnel junction or by a mechanical form of electrical interconnect. Tandem solar cells overcome a fundamental limitation of single cells, which are limited to absorption of a narrower band of wavelengths. The fundamental efficiency limitation in a single solar cell results from the trade off between a low bandgap, which maximizes light absorption and hence the output current, and a high band gap, which maximizes output voltage. In tandem cells having two or more series-connected cells with different bandgaps the top cell converts the high energy photons (UV and visible photons) and the bottom cell made of a material with smaller bandgap converts transparency losses of the the top cell.
In recent years GaInP/GaAs tandem solar cells with AMO (sunlight incidence in space=1.35 kW/m.sup.2) efficiencies in excess of 25% AMO have been reported (Bertness et al, Proc. 24th IEEE PVSC, 1994, pp. 1671-1678). Driven by a demand for satellites with more on-board power the technology has rapidly become one of the industry standards, produced by major photovoltaic (PV) manufacturers. By the end of 1998 these cells were expected to represent almost one third of III-V-semiconductor space solar cell market, according to Meyer and Metzger ("Compound Semiconductors," special issue on current status of the semiconductor industry, 1997, pp. 40-41).
Efficiencies above 30% will be available by substituting the GaAs cell (band gap of 1.42 eV) with a cell that efficiently absorbs lower energy photons and is crystalographically lattice-matched to commonly used Ge or GaAs substrates. But, most common semiconductors having bandgaps in the range of interest, such as ternary In.sub.X Ga.sub.1-x As alloys, are lattice-mismatched to GaAs. (Lattice mismatch for a 1.2eV In.sub.0.2 Ga.sub.0.8 As is about 1.4%). In order to avoid defect generation and minority carrier performance degradation, only very thin layers (a few hundred Angstroms thick) of these materials can be grown on GaAs. The thin layers are not thick enough for the fabrication of efficient conventional cells. As a result the efficiency and radiation hardness of the existing tandem devices are mainly limited by the photocurrent output and the radiation induced degradation of the GaAs bottom cell. Highest efficiencies are achieved by reducing the thickness (and performance) of the top GaInP cell to below 1 micron, to favor higher photon flux in the bottom GaAs
What is needed is a bottom cell of a tandem solar cell that is capable of producing increased electrical current from the bottom or GaAs cell by absorbing photons having insufficient energy to be absorbed in the top or GaInP cell. The bottom cell should have characteristics such that reducing the thickness of the GaInP cell is not required and it should be lattice-matched to a GaAs or Ge substrate so as to avoid crystalline defects in the cell.